Cell Respiration


  • Cell respiration is the controlled release of energy from organic compounds in cells to form ATP (adenosine triphosphate)
  • The main organic compound used for this process is glucose, however lipids and proteins can also be broken down via modified processes 
  • There are two types of cell respiration:
    • Anaerobic respiration does not require oxygen to occur, but results in only a small yield of ATP via a process which occurs in the cytoplasm
    • Aerobic respiration does require oxygen to occur, but results in a much larger yield of ATP via a process which occurs in the mitochondria
  • Both processes begin with the breakdown of glucose via glycolysis


  • Glycolysis is the first stage of cell respiration and involves the breakdown of glucose (6C) into two molecules of pyruvate (3C)
  • It is an anaerobic reaction (does not require the presence of oxygen) and occurs in the cytoplasm
  • It requires 2 molecules of ATP to occur, but produces 4 molecules of ATP as a result 
    • This results in a net gain of 2 ATP molecules being made by glycolysis
  • It also releases hydrogen which is taken up by the hydrogen carrier NAD+ to form NADH (x2)
    • Glycolysis cannot occur without a ready supply of NAD+
  • Overall:  One molecule of glucose results in 2 pyruvate, 2 NADH and 2 ATP (net gain)

Anaerobic Respiration

  • Anaerobic respiration occurs in the cytoplasm in the absence of oxygen
  • Aerobic respiration uses NADH to produce large amounts of ATP in the presence of oxygen (releasing NAD+)
    • In the absence of oxygen, the NADH remains idle and cannot be used to release NAD+ 
    • This means glycolysis cannot occur (no spare NAD+) and no ATP can be produced
  • Anaerobic respiration converts pyruvate into a temporary by-product in a reaction that uses NADH to release NAD+
  • This allows for low amounts of ATP to be produced via glycolysis (as there is now free stock of NAD+)
    • In animals:  Pyruvate is converted into lactic acid (lactate)
    • In plants and fungi:  Pyruvate is converted into ethanol and CO2 (this process is also known as fermentation)
  • This reaction is reversible, so when oxygen is present, these by-products can be converted back into pyruvate for use in aerobic respiration 
  • Overall:  Two molecules of ATP are produced (via glycolysis) and pyruvate is converted into lactic acid (animals) or ethanol and CO2 (plants)

Anaerobic Respiration


  • Aerobic respiration occurs in the mitochondrion
  • Mitochondria are double-membrane organelles where the inner membrane is highly folded into finger-like projections called cristae (increased SA)
  • The area within the inner membrane is known as the matrix, while the space between the two membranes is known as the intermembrane space

Structure of a Mitochondrion

                2D Representation                                                           3D Representation                                                                     Electron Micrograph

Aerobic Respiration

  • Aerobic respiration takes place in the mitochondria, using the pyruvate produced via glycolysis
  • It produces large amounts of ATP in the presence of oxygen via three main processes:

The Link Reaction

  • Pyruvate is transported from the cytosol to the mitochondrial matrix in a reaction that produces (one) NADH 
  • The pyuvate loses a carbon (as CO2) and the remaining two carbons are complexed with coenzyme A (CoA) to form acetyl CoA

The Krebs Cycle

  • In the matrix, acetyl CoA combines with a 4C compound to form a 6C compound
  • Over a series of reactions the 6C compound is broken back down into the original 4C compound
  • These reactions result in the formation of 2 CO2 molecules, 1 ATP molecule and multiple hydrogen carriers (specifically 3 NADH and 1 FADH2

The Electron Transport Chain

  • The hydrogen carriers (NADH and FADH2) provide electrons to the electron transport chain on the inner mitochondrial membrane
  • As the electrons cycle through the chain they lose energy, which is used to translocate H+ ions to the intermembrane space (creating a gradient)
  • The hydrogen ions return to the matrix through the transmembrane enzyme   ATP synthase, producing multiple ATP molecules (via chemiosmosis)
  • Oxygen acts as a final electron acceptor for the electron transport chain,  allowing further electrons to enter the chain
  • Oxygen combines the electrons with H+ ions to form water molecules
  • The electron transport chain produces the majority of the ATP molecules produced via aerobic respiration (~32 out of 36 ATP molecules)

Overview of Chemiosmosis

Summary of Cell Respiration

Anaerobic Respiration:  Occurs in the cytoplasm in the absence of oxygen and produces 2 molecules of ATP (net gain)

  • Animals:  Glucose is converted into lactic acid (and 2 molecules of ATP)
  • Plants and Fungi:  Glucose is converted into ethanol and carbon dioxide (and 2 molecules of ATP)

Aerobic Respiration:  Occurs in the cytoplasm (glycolysis) and mitochondria in the presence of oxygen

  • Overall:  Glucose (and oxygen) is converted into water, carbon dioxide and ATP (net gain of 36 - 38 ATP molecules)

Anaerobic versus Aerobic Respiration